While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the organic EL element encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material or dopant, which results in an efficiency improvement and allows color tuning.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding these developments, there are continuing needs for organic EL device components, such as dopants, that will provide high luminance efficiencies combined with high color purity and long lifetimes.
A useful class of dopants is derived from the 5,6,5-tricyclic pyrromethene-BF2 complexes and disclosed in U.S. Pat. No. 5,683,823; JP 09 289,081A; JP 11 097,180A, and U.S. Patent Publication 2003-0198829-A1. These materials are characterized by typically narrow emission spectra, which may result in attractively high color purity. However, the green-emitting unsubstituted or alkyl substituted pyrromethene-BF2 complexes exhibit relatively low quantum efficiencies of electroluminescence. In order to achieve highly efficient OLEDs, one needs to use phenyl rings as substituents thereby extending the conjugated π-system. As a result, the emission wavelength typically becomes red-shifted yielding a reddish amber color, which is the shortest wavelength light that can be emitted by pyrromethene-BF2 complexes with good efficiency. In simple terms, luminance efficient green OLEDs do not appear to be conveniently obtained with pyrromethene BF2 complexes used as dopants.
It is a problem to be solved to provide a light-emitting compound for a light-emitting layer of an OLED device that exhibits improved luminance efficiency and a desirable hue.